Addiction to heroin is a major social problem, affecting over a million people in the United States. In the body and brain, heroin is hydrolyzed to morphine, which acts at the mu opioid receptor and results in a euphoric effect, thus conferring the reinforcing properties of the drug and contributing to addiction. Drug addiction is a complex process, thought to result from the interaction of social, environmental, and biological factors including a genetic component. In our original proposal, we hypothesized that genetic polymorphisms might exist in the mu opioid receptor and alter receptor function, contributing to variation in individual susceptibility to heroin abuse. During the current funding period we have observed five different single nucleiotide polymorphisms (SNPs); two of these SNPs were relatively common (10.5 percent and 6.6 percent allelic frequency), and both showed differential distributions among ethnic groups. One appeared to exert a protective effect against opioid addiction in one of the ethnic groups; it also altered beta-endorphin binding affinity and agonist potency. The other common variant occurred with a significantly higher frequency in former heroin addicts, suggesting a genetic predisposition for opioid dependence. In this competing renewal application, we propose to extend our study of the clinical and functional significance of genetic variation in the human mu opioid receptor. We will examine a larger number of study subjects, and determine the allele distribution in former heroin addicts and controls among different ethnic groups. We will also examine their impact on receptor modulation of neuronal Ca2+ channels and inhibition of adenylyl cyclase activity. Furthermore, we will determine the effects of the mu receptor polymorphisms on the cellular responses to chronic morphine treatment, since the primary problems that develop during heroin addiction come from prolonged exposure to the opioid drug. Results from the proposed study in this renewal application will provide valuable information in two areas: First of all, by studying the effects of sequence variants on the cellular function of the mu receptor, we will understand how genetic polymorphisms impact on receptor activity and neuronal excitability. Furthermore, by determining distribution of these genetic variations between opioid-dependent individuals and normal controls, we will start to appreciate the role of genetic polymorphism is predisposition for or protection against opioid dependence.